Damascene processing is a method for forming metal lines on integrated circuits. It involves formation of inlaid metal lines in trenches and vias formed in a dielectric layer (inter metal dielectric). Damascene processing is often a preferred method because it requires fewer processing steps than other methods and offers a higher yield. It is also particularly well-suited to metals such as copper that cannot be readily patterned by plasma etching.
In a typical Damascene process flow, metal is deposited onto a patterned dielectric to fill the vias and trenches formed in the dielectric layer. The resulting metallization layer is typically formed either directly on a layer carrying active devices, or on another metallization layer. A stack of several metallization layers can be formed using Damascene processing.
Deposition of metal in Damascene processing is often accomplished by electroplating methods. During electroplating, the wafer substrate is electrically biased to serve as a cathode. The cathode is immersed into the plating solution containing metal ions, which are reduced at the cathode surface to form a layer of electrodeposited metal. Electroplating methods require that an electrical contact is made with the wafer substrate. This is typically accomplished using metal seed layers pre-deposited on the wafer surface. In order to achieve good quality of metal deposition during electroplating, a continuous seed layer of metal should reside on the surface of the substrate. Seed layers can be deposited by several methods, of which PVD and electroless deposition are most notable.
As the dimensions of features in integrated circuits continue to shrink, deposition of continuous seed layers becomes increasingly challenging. Currently there is a need for new methods of seed layer formation, as well as for apparatus that can be used for seed layer deposition.